1. Field of the Invention
The present invention relates to a transmitting apparatus, a receiving apparatus, and an information communication method, and more particularly, to an effective technique applied to a retransmission control technique, etc. in transmitting and receiving apparatuses that configure a wireless communication system.
2. Description of the Related Art
The demand for realizing a high-speed and large-capacity communication has been increasing in order to implement services such as transmission/reception of large-capacity data except for voice, and image data of high definition in a mobile communication field typified by a cellular phone, etc.
A technique for compensating for an error in a wireless transmission section is essential to a current wireless communication, which takes a high-speed and large-capacity communication into account, in order to improve its throughput. An automatic retransmission control method (ARQ: Automatic Repeat reQuest) exists as an important error compensation technique for improving a throughput. FIG. 1 shows a conceptual drawing showing the action of ARQ that is a conventional retransmission control method. With ARQ, error detection is made for each packet by using an error detection code such as a CRC (Cyclic Redundancy Check) code, which is appended to a transmission packet, the transmission of the next packet is requested by returning an ACK signal to a transmitting side if a received packet does not have an error, or the retransmission of the packet is requested by returning a NACK (Not-ACKnolwedge) signal if the received packet has an error.
HARQ (Hybrid-ARQ) exists as one type of ARQ. Here, a description is provided by taking as an example HARQ using a packet synthesis, which is referred to in Non-patent Document 1, among HARQ methods. Hereinafter, HARQ is assumed to indicate HARQ using a packet synthesis in this paragraph. FIG. 2 is a conceptual drawing showing the principle of the HARQ method. With the HARQ, error detection is made for each frame by using a CRC, which is appended to the end of a transmission frame, or the like after an error correction is made. If an error is detected, a NACK signal is returned to a transmitting side, and the retransmission of the entire frame is requested. At this time, a receiving side stores in a buffer the reception signal of the frame from which the error is detected. With the HARQ, the reception result of a retransmitted frame and the reception signal of the initially transmitted frame are synthesized. As a result, the reception characteristic can be further improved by using soft-determination information of the initial frame.
With conventional retransmission control methods such as HARQ, a retransmission is controlled by using an acknowledgment wait time called RTT (Round Trip Time). RTT is the sum of a time during which a packet arrives from a transmitting side to a receiving side, a time during which whether or not a packet is successfully received by the receiving side is determined, and a time during which ACK/NACK, which indicates an acknowledgement result, arrives from the receiving side to the transmitting side. A predetermined value is used as the RTT. The transmitting side transmits new data after a predetermined RTT elapses from the transmission of the initial data if an ACK signal arrives from the receiving side within the predetermined RTT, or retransmits the data after the RTT elapses if a NACK signal arrives. Or, if ACK or NACK does not arrive within the RTT, the transmitting side retransmits the packet to the receiving side.
In a next-generation mobile communication, it is anticipated that (objective 1) a seamless connection with a wired network, and (objective 2) a large-capacity wireless transmission at a transmission rate as high as 1 Gbps will be realized. To achieve (objective 1), TCP (Transmission Control Protocol), which is widely used as a transmission protocol of a wired network, must be implemented in a wireless network. TCP fundamentally observes congestion within a network, and gradually increases a transmission rate if congestion does not occur, or decreases the transmission rate to a minimum if congestion occurs. Whether or not congestion occurs is determined based on a packet loss.
The following problem is posed when TCP is used for a wireless system. TCP determines that a packet loss occurs if ACK is not returned from a receiving side to a transmitting side within a preset timeout period. However, a delay time is long in a wireless environment compared with a wired environment, leading to an increase in the probability that the timeout period of TCP expires and a packet loss is determined to occur. As a result, TCP determines that the packet loss is caused by congestion, and strictly restricts the inflow amount of data into a network. Therefore, a system throughput tends to be degraded if TCP is applied to a wireless environment. This is a serious problem when a large-capacity transmission at high speed is realized (objective 2). To improve degradation in a throughput, an improvement in TCP protocol, and a reduction in a delay time in a wireless environment are considered to be possible solutions. The latter is focused here.
A delay that occurs at the time of a retransmission process is considered as one factor of a processing delay in a wireless environment. With a conventional method for controlling a retransmission by using RTT, a considerable amount of time is required to transmit/retransmit new data depending on the value of RTT, and a retransmission delay increases if the retransmission is repeated, leading to degradation in a system throughput.
Patent Document 1 discloses as a conventional technique the technique for ensuring a sufficient call connection ratio while suitably maintaining the traffic of a general calling channel by varying the number of times that a call signal is retransmitted from a base station to a mobile station for each of a plurality of wireless areas having different line qualities such as fading, interference, etc. However, this document does not refer to the above described technical problem in the retransmission control of communication data itself.
Non-patent Document 1: D. Chase, “Code Combining—A Maximum-Likelihood Decoding Approach for Combining an Arbitrary Number of Noisy Packets”, IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. COM-33, NO. 5, MAY 1985
Patent Document 1: Japanese Published Unexamined Patent Application No. H10-13331